Process for producing fluoroelastomers

ABSTRACT

Fluoroelastomers having copolymerized units of vinylidene fluoride and at least one other fluorinated monomer are prepared in aqueous suspension using an initiator comprising a solution of an oil soluble peroxide in a water-soluble hydrocarbon solvent.

This is a continuation, of application Ser. No. 08/525,512 filed Nov.28, 1995, now abandoned.

BACKGROUND OF THE INVENTION

This invention pertains to a novel process for the production of afluoroelastomer.

In particular, it pertains to a process for the production of afluoroelastomer in which the vulcanized product has excellent heatresistance, oil resistance, and chemical resistance and wherein theMooney viscosity at the same molecular weight is lower than that of afluoroelastomer prepared by the emulsion polymerization method of theprior art, the permanent compression set is low, the mold-releasingproperties are excellent, and mold contamination is insignificant. Thefluoroelastomer is produced by carrying out suspension polymerization inan aqueous medium. It is a safe and efficient process because no ozonelayer-depleting chlorofluorocarbon solvents are used and recovery ofmonomers and solvents is simple.

Fluoroelastomers having excellent heat resistance, oil resistance, andchemical resistance have been used widely for containers, sealingmaterials, and hoses which are used under severe conditions in theindustry. Industrially useful fluoroelastomers include fluoroelastomerdipolymers comprising vinylidene fluoride (VF₂) and hexafluoropropylene(HFP) units and fluoroelastomer terpolymers comprising VF₂, HFP, andtetrafluoroethylene (TFE) units.

Production of such fluoroelastomers by emulsion polymerization,suspension polymerization, and solution polymerization methods is knownin the art. In the suspension polymerization method, polymerization iscarried out by dispersing a monomer, or organic solvent with a monomerdissolved therein, in water and using an oil soluble organic peroxide.This method is preferable on an industrial scale because the posttreatment is simple, and the fluoroelastomer prepared has excellentthermal stability, workability, and mechanical properties as disclosedin U.S. Pat. Nos. 3,801,552 and 4,985,520.

In the case of suspension polymerization, halogenated hydrocarbons suchas, for example, trichlorotrifluoroethane, are used as solvent media.Such halogenated hydrocarbons improve affinity between fluoroelastomerproduct and monomer, and permit the monomer to be dissolved in thepolymer particles. However, after polymerization, it is difficult torecover the solvent, e.g. trichlorotrifluoroethane, and monomer. Inaddition, halogenated hydrocarbons such as chlorofluorocarbons arealleged to be ozone layer depleting compounds and their use is graduallybeing curtailed internationally.

In order to prepare a high molecular weight fluoroelastomer, the use ofhalogenated hydrocarbons having low chain transfer reactivity in thepolymerization reaction, such as CFC-113 (CCl₂ FCClF₂) and HC-141b (CH₃CFCl₂), is optimal. Because the chain transfer reactivity of hydrocarbonsolvents is generally high, preparation of high molecular weightfluoroelastomers in the latter solvents has not been utilized.

There is a method for carrying out suspension polymerization in theabsence of halogenated hydrocarbon by simply adding an oil solubleorganic peroxide Japanese Patent Application Publication Kokai Nos.3-207,701 (1991) and 3-247,608 (1993)!. In this method, the oil solubleorganic peroxide is not diluted. Consequently, the safety at the time ofhandling, for example in transportation, is a problem, and this methodis not practical.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a safe and efficientprocess for the production of a fluoroelastomer which uses no ozonelayer depleting chlorofluorocarbon type solvent by carrying outsuspension polymerization in an aqueous medium. It is a furtherobjective of the invention to provide a process which producesfluoroelastomer product which, when vulcanized, has excellent heatresistance, oil resistance, and chemical resistance, low compressionset, excellent mold-releasing properties, and exhibits insignificantmold fouling. Further, in the product so produced the Mooney viscosityat the same molecular weight is lower than that of a fluoroelastomerprepared by the emulsion polymerization method of the prior art.

In particular, the present invention is directed to a suspension processfor producing a fluoroelastomer having copolymerized units of vinylidenefluoride monomer and at least one other copolymerizable fluorinatedmonomer which comprises

(A) dispersing said monomers in an aqueous medium containing 0.001-3parts by weight of a suspension stabilizer per 100 parts of the aqueousmedium and 0.001-5 parts by weight of an oil soluble organic peroxidepolymerization initiator solution per 100 parts of the aqueous medium;and

(B) polymerizing the resultant dispersion at a temperature of 45° C.-70°C.;

wherein said oil soluble organic peroxide polymerization initiatorsolution consists essentially of 0.1-75 wt. % of an oil soluble organicperoxide in a water-soluble hydrocarbon solvent and said water-solublehydrocarbon solvent contains no halogen atom and is represented bycompounds of the formulas R₁ OH, R₂ COOR₁, and R₁ COR₃, where R₁ and R₃are methyl or t-butyl groups, and R₂ is hydrogen, a methyl group or at-butyl group.

In a preferred embodiment, an iodine compound may additionally bepresent. Suitable iodine compounds are represented by the generalformula: RI_(n), where R is a hydrocarbon group having 1-3 carbon atomsor a saturated or unsaturated fluorohydrocarbon orchlorofluorohydrocarbon group having less than 6 carbon atoms, and n is1 or 2, and the iodine compound is added in the amount of 0.005-5 partsby weight per 100 parts by weight of the aqueous medium and dispersed.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, fluoromonomers copolymerizable withVF₂ include, for example, HFP, TFE, and perfluoroalkyl perfluorovinylether (PAVE). Specific examples of PAVE include, perfluoromethylperfluorovinyl ether, perfluoroethyl perfluorovinyl ether, andperfluoropropyl perfluorovinyl ether.

In the case of fluoroelastomer dipolymers comprising VF₂ and HFP unitsand fluoroelastomer terpolymers comprising VF₂, HFP, and TFE units,which are examples of the most preferred mode of this invention, theratio of the VF₂ units to the HFP units by weight is generally in therange of 40:60 to 80:20 and preferably in the range of 55:45 to 75:25 inthe dipolymer fluoroelastomers or 45:55 to 70:30 in the terpolymerfluoroelastomers. Furthermore, the TFE unit content in the above ternaryfluoroelastomers is in the range of 1-35 wt %, preferably 5-25 wt %.

Preferred examples of fluoroelastomers containing PAVE contain, forexample, 10-85 mol % of VF₂ units, 3-80 mol % TFE units and 2-50 mol %PAVE units.

Fluoroelastomers having the above monomer unit contents exhibiteffective rubber elasticity. However, if the ratio is outside the aboverange, the polymers obtained exhibit resin properties and, consequently,they are not suitable for accomplishing the objectives of thisinvention.

According to the process of this invention, (1) a monomer mixture of adesired composition (initial monomers charged) is dispersed in anaqueous medium containing a suspension stabilizer in an amount of0.001-3 parts by weight per 100 parts by weight of the aqueous medium,and, if necessary, an iodine compound represented by the generalformula: RI_(n), where R is a hydrocarbon group having 1-3 carbon atomsor a saturated or unsaturated fluorohydrocarbon orchlorofluorohydrocarbon group having less than 6 carbon atoms, and n is1 or 2, is also added while stirring mechanically, (2) the liquidtemperature is maintained in the range of 45° C.-70° C., preferably 50°C.-60° C., and (3) the suspension polymerization reaction is carried outby adding a polymerization initiator solution prepared by diluting awater soluble organic peroxide in an amount of 0.001-5 parts by weightper 100 parts by weight of the aqueous medium to a concentration of0.1-75 wt % with a water soluble hydrocarbon solvent wherein hydrocarbonsolvent wherein the hydrocarbon contains no halogen atoms and isrepresented by the general formulas R₁ OH, R₂ COOR₁, or R₁ COR₃, whereR₁, R₂, and R₃ are the same as those mentioned above.

The polymerization temperature is maintained in the range of 45° C.-70°C. If it is below 45° C., the rate of polymerization is too slow, whileif it is above 70° C., suspended particles of the polymer formed becomesticky and are liable to cause blocking. It thus becomes difficult tomaintain a stable state of suspension during the polymerizationreaction.

The polymerization pressure is in the range of 5-50 kg/cm² G, preferably8-30 kg/cm² G. The desired polymerization pressure is achieved byadjusting the amount of monomer initially charged, and after thereaction is initiated, the monomer mixture (supplemental monomers) isadded in such a way that pressure is maintained at a constant desiredlevel to carry out the polymerization reaction. The polymerizationpressure is set in the above range because if it is below 5 kg/cm² G,the monomer concentration in the polymerization reaction system is toolow to obtain a satisfactory reaction rate. In addition, the molecularweight does not increase sufficiently. If the pressure is above 50kg/cm² G, the amount of monomer liquefied is increased, thereby merelyincreasing the amount of monomer which is not consumed, thereby invitingpoor production efficiency.

The amount of polymer formed is approximately equal to the amount ofsupplemental monomer charged, and is in the range of 10-300 parts byweight of the polymer per 100 parts by weight of the aqueous medium,preferably in the range of 20-250 parts by weight of the polymer.

The degree of polymer formation is set in the above range because ifless than 10 parts by weight, productivity is significantly low, whileif it is above 300 parts by weight, the solids content becomes too highfor satisfactory stirring.

Oil soluble organic peroxides which may be used in this inventioninclude, for example, dialkylperoxydicarbonates, such asdiisopropylperoxydicarbonate (IPP), di-sec-butylperoxydicarbonate,di-sec-hexylperoxydicarbonate, di-n-propylperoxydicarbonate, anddi-n-butylperoxydicarbonate; peroxyesters, such astert-butylperoxyisobutyrate and tert-butylperoxypivalate;diacylperoxides, such as dipropionyl peroxide; anddi(perfluoroacyl)peroxides or di(chlorofluoroacyl)peroxides such asdi(perfluoropropionyl)peroxide anddi(trichloro-octafluorohexanoyl)peroxide. The use of dialkylperoxydicarbonates is preferably, and the use of IPP is most preferred.These oil soluble organic peroxides may be used alone or as a mixture oftwo or more types. The amount to be used is selected generally in therange of 0.001-5 parts by weight per 100 parts by weight of the aqueousmedium, preferably 0.01-3 parts by weight.

Oil soluble organic peroxides are associated with a risk of explosionwhen heated or on impact, and, consequently, they are difficult totransport unless they are diluted with a solvent. Previously, suchorganic peroxides were diluted with halogenated hydrocarbon solvents andused in suspension polymerization to prepare fluoroelastomers. It wasbelieved that use of a hydrocarbon as a diluent was impractical becauseradical abstraction of hydrogen atoms from the solvent at the time ofpolymerization would occur, rendering it difficult to obtain a highmolecular weight product and depressing the polymerization reactionrate. However, if oil soluble organic peroxides are diluted with a watersoluble hydrocarbon solvent containing no halogen atoms, represented bythe following general formulas R₁ OH, R₂ COOR₁, or R₁ COR₃, where R₁ andR₃ are methyl or t-butyl groups, and R₂ is hydrogen, a methyl group or at-butyl group, it is possible to produce a fluoroelastomer of lowmolecular weight to high molecular weight safely and efficiently. At thesame time, the recovery of monomers and solvent is not difficult. Thehydrocarbon solvents of the present invention do not have substantialadverse effects on the polymerization reaction because the chaintransfer reactivity of these hydrocarbon solvents is relatively small.At the same time, they are soluble in the aqueous medium. Further, onlytrace amounts are contained in micelles comprised of the monomers andoil soluble organic peroxide in which the polymerization reactionoccurs. Also, the solvent or monomers diffuse into the polymer formedwith difficulty and consequently, the recovery of the solvent andmonomers is not difficult.

Specific examples of water-soluble, non-halogenated hydrocarbon solventsof this invention are methanol, tert-butyl alcohol, methyl formate,tert-butyl formate, methyl acetate, tert-butyl acetate, methyl pivalate,tert-butyl pivalate, acetone, methyl tert-butyl ketone, anddi-tert-butyl ketone. The use of methanol, tert-butyl alcohol, methylacetate, or tert-butyl acetate is preferable and methyl acetate ortert-butyl acetate are most preferred. These solvents may be used aloneor as a combination of two or more types. The solvent is used to dilutean oil soluble organic peroxide to a concentration of 0.1-75 wt %,preferably 1-60 wt % . If the concentration is over 75 wt %, the organicperoxide concentration is too high for safe transportation. On the otherhand, if it is below 0.1 wt %, the concentration is so low that theamount of solvent to be recovered becomes undesirably high.

When producing peroxide-curable fluoroelastomer according to the processof the present invention, it is particularly useful to introduce aniodine compound represented by the general formula RI_(n), where R is ahydrocarbon group having 1-3 carbon atoms or a saturated or unsaturatedfluorohydrocarbon or chlorofluorohydrocarbon group having less than 6carbon atoms, and n is 1 or 2. The iodine compound introduces sites inthe polymer which can take part in crosslinking reactions. In theformula, n must be 1 or 2 because if n were 3, the fluoroelastomerproduced would have a three-dimensional structure providing poorworkability. The iodine compound is selected from those which do notdecompose or lose their activity under the polymerization conditionsused. Specific iodine compounds include monoiodomethane, isdiiodomethane, 1-iodoethane, 1,2-diiodoethane, 1-iodo-n-propane,isopropyl iodide, 1,3-diiodo-n-propane, 1,4-diiodoperfluoro-n-butane,1,6-diiodofluoro-n-hexane, and1,5-diiodo-2,4-dichloroperfluoro-n-pentane. The use of diiodomethane ismost preferred because of its polymerization reactivity, vulcanizationreactivity, and availability. The iodine compounds may be used alone oras a combination of two or more types.

In the case of copolymerization of VF₂ and a copolymerizablefluoroolefin in the presence of an iodine compound represented by thegeneral formula, the carbon iodine bond of the iodine compound reactswith the radical formed, conventional telomerization occurs, and aniodine atom is introduced at the polymer end. The amount of iodinecompound used is in the range of 0.005-5 parts by weight, preferably0.05-3 parts by weight.

Suspension stabilizers useful in the present invention include, forexample, methyl cellulose, carboxymethyl cellulose, bentonite, talc, anddiatomaceous earth. Methyl cellulose is preferred. These suspensionstabilizers may be used alone or as a combination of two or more types.The amount utilized is generally in the range of 0.001-3 parts byweight, preferably 0.01-1 part by weight per 100 parts by weight of theaqueous medium.

Polymerization times in the range of from 3-50 hours, as in conventionfluoroelastomer preparations, are employed in this invention. The amountof fluoroelastomer formed is roughly equal to the amount of monomercharged and the composition of monomer charged is roughly the same asthat of the fluoroelastomer to be prepared.

The monomer composition of the initial charge and that of supplementalmonomer added are determined by gas chromatography. The monomercomposition in the fluoroelastomer prepared is determined by dissolvingthe fluoroelastomer in acetone and carrying out ¹⁹ F-NMR analysis.

The fluoroelastomers prepared by this invention are generally vulcanizedand molded before use. Suitable vulcanization methods employ polyol andpolyamine compounds as curatives. Vulcanization with a polyol compoundis especially advantageous because compression set resistance isimproved further. Fluoroelastomers prepared using an iodine compound ofthis invention can be vulcanized with a polyol or polyamine compound butperoxide vulcanization using an organic peroxide is also possible. Whenperoxide curatives are used, resistance to chemicals such as acids orbases is markedly improved.

When vulcanization using a polyol compound is employed, the methodinvolves mixing a fluoroelastomer of the invention with (a) apolyhydroxy aromatic compound, (b) a vulcanization accelerator, (c)divalent metal hydroxide and/or divalent metal oxide, and, if necessary,other components, and kneading the mixture on a rubber mill or in aBanbury mixer. The resultant mixture is then placed in a mold,pressurized to effect primary vulcanization (press cure) and,subsequently, a secondary vulcanization (post cure) is carried out. Ingeneral, the press cure is effected at 100° C.-200° C. at 20-100 kg/cm²for 10-180 minutes. The post cure is carried out at 150° C.-300° C. for0-30 hours. Furthermore, the post cure step may be omitted in somecases.

Examples of component (a) polyhydroxy aromatic compounds includebisphenol AF, bisphenol A, bisphenol S, dihydroxybenzophenone,hydroquinone, 4,4'-thiodiphenol, and their metal salts. Bisphenol AF ismost preferred. The amount used is generally in the range of 0.1-10parts by weight, preferably 0.6-5 parts by weight per 100 parts byweight of the fluoroelastomer. The particular component (a) range isselected because if less than 0.1 part by weight is present,vulcanization is incomplete, while, on the other hand, if more than 10parts by weight is used, elasticity is sacrificed. The polyhydroxyaromatic compounds may be used alone or as a combination of two or moretypes.

Component (b) vulcanization accelerators include phosphonium salts,ammonium salts, iminium salts, sulfonium salts, and aminophosphinederivatives. Specific examples include benzyltriphenylphosphoniumchloride, methyltriphenylphosphoniummethylmethane phosphonate,tetrabutylammonium fluoride, tetrabutylammonium bromide,8-benzyl-1,8-diazabicyclo(5,4,0)-undecenonium chloride, andbis(benzyldiphenylphosphine)iminium chloride. The use ofbenzyltriphenylphosphonium chloride,8-benzyl-1,8-diazabicyclo(5,4,0)-undecenonium chloride, andbis(benzyldiphenylphosphine)iminium chloride is preferable. The amountemployed is generally in the range of 0.05-2 parts by weight, preferably0.1-1 part by weight, per 100 parts by weight of the fluoroelastomer. Ifcomponent (b) is present at levels below 0.05 part by weight, thevulcanization rate is too slow. If it is present in amounts above 2parts by weight, the compression set becomes unacceptably poor. Theaccelerators may be used alone or as a combination of 2 or more types.

Component (c) divalent metal hydroxides and/or divalent metal oxidesinclude, for example, oxides and hydroxides of magnesium, calcium, zinc,and lead. The use of oxides and hydroxides of magnesium and calcium ispreferred. The amount compounded is generally in the range of 1-30 partsby weight, preferably 2-20 parts by weight per 100 parts by weight ofthe fluoroelastomer. The particular component (c) range is selectedbecause if less than 1 part by weight is used, vulcanization is notcomplete. On the other hand, if more than 30 parts of component (c) ispresent, the compression set becomes poor. The component (c) compoundsmay be used alone or as a combination of 2 or more types.

In addition, if necessary, other components, for example, fillers suchas carbon black, Austin black, graphite, silica, clay, diatomaceousearth, talc, wollastonite, calcium carbonate, calcium silicate, calciumfluoride, and barium sulfate; processing aides such as higher fatty acidesters, fatty acid calcium salts, fatty acidamides, low molecular weightpolyethylene, silicone oil, silicone grease, stearic acid, sodiumstearate, calcium stearate, magnesium stearate, aluminum stearate, andzinc stearate; coloring agents such as titanium white and iron red maybe used as compounding additives. The amount of such filler compoundedis generally in the range of 0.1-100 parts by weight, preferably 1-60parts by weight, per 100 parts by weight of the fluoroelastomer. Thisrange is selected because if the filler is present in amounts of lessthan 0.1 part by weight, there is no effect, while, on the other hand,if greater than 100 parts by weight are used, elasticity is sacrificed.The amount of processing aid compounded is generally less than 10 partsby weight, preferably less than 5 parts by weight, per 100 parts byweight of the fluoroelastomer. If the amount used is above the limit,heat resistance is adversely affected. The amount of a coloring agentcompounded is generally less than 50 parts by weight, preferably lessthan 30 parts by weight per 100 parts by weight of the fluoroelastomer.If greater than 50 parts by weight is used, compression set suffers.

In the case of diaphragm applications, for example elongation is morecritical than compression set resistance. Thus, vulcanization with apolyamine compound is preferably employed. The same vulcanizationconditions as those employed in the vulcanization methods utilizingpolyol compounds may be used. Polyamine vulcanization is carried outwith the above (a) polyhydroxyaromatic compounds and instead of a (b)vulcanization accelerator, a (d) polyamine compound selected from thegroup of, for example, hexamethylenediamine, hexamethylenediaminecarbamate, ethylenediamine carbamate,N,N-dicinnamylidene-1,6-hexamethylenediamine, and4,4'-bis(aminocyclohexyl)methane carbamate, is employed. The amount ofthe component (d) compounded is generally in the range of 0.1-10 partsby weight, preferably 0.5-5 parts by weight per 100 parts by weight ofthe fluoroelastomer. The particular range of component (d) is selectedbecause if less than 0.1 part by weight is used, vulcanization of moldedgoods is incomplete, while, on the other hand, if greater than 10 partsby weight is used, elasticity is compromised. The polyamine compoundsmay be used singly or in combinations of two or more types.

The peroxide vulcanization method can be exemplified as follows. To afluoroelastomer of this invention is added (a) organic peroxide, (f)polyfunctional unsaturated compound, if necessary, one of theabove-described (c) divalent metal hydroxides and/or divalent metaloxides and any other components. The mixture is kneaded using a rubbermill or Banbury mixer. Subsequently, the mixture is placed in a mold,pressurized to effect primary vulcanization (press cure), and then asecondary vulcanization (post cure) is carried out. In general, thefirst vulcanization is effected at 100° C.-200° C. under 20-300 kg/cm²for 5-30 minutes, and the post cure is carried out at 100° C.-200° C.for 0-30 hours. The post cure may be omitted in some cases.

Component (e) organic peroxides suitable for use include organicperoxides which generate peroxide radicals under the vulcanizationconditions, such as 1,1-bis(t-butylperoxy)-3,5,5-trimethylcyclohexane;1,1-bis(t-butylperoxy)cyclohexane; 2,2-bis(t-butylperoxy)octane;n-butyl-4,4-bis(t-butylperoxy)valerate; 2,2-bis(t-butylperoxy)butane;2,5-dimethylhexane-2,5-dihydroxyperoxide; di-t-butyl peroxide;t-butylcumyl peroxide; dicumyl peroxide; α,α'-bis(t-butylperoxy-m-isopropyl)benzene;2,5-dimethyl-2,5-di(t-butylperoxy)hexane;2,5-dimethyl-2,5-di(t-butylperoxy)hexene-3; benzoyl peroxide,t-butylperoxybenzene; 2,5-dimethyl-2,5-di(benzoylperoxy)-hexane;t-butylperoxymaleic acid; and t-butylperoxyisopropylcarbonate. Preferredexamples of the component (e) peroxides include2,5-dimethyl-2,5-di(t-butylperoxy)hexane, dicumyl peroxide, and α,α'-bis(t-butylperoxy-m-isopropyl)benzene. The amount compounded isgenerally in the range of 0.05-5 parts by weight, preferably in therange of 0.1-3 parts by weight per 100 parts by weight of thefluoroelastomer. The particular component (c) range is selected becauseif the peroxide is present in an amount of less than 0.05 parts byweight, the vulcanization rate is insufficient and causes poor moldrelease. On the other hand, if the peroxide is present in amounts ofgreater than 5 parts by weight, the compression set becomes unacceptablyhigh. In addition, the organic peroxides may be used singly or incombinations of two or more types.

Specific examples of the component (f) polyfunctional unsaturatedcompound are triallyl cyanurate, trimethacryl isocyanurate, triallylisocyanurate, triacryl formal, triallyl trimellitate, N,N'-m-phenylenebismaleimide, diallyl phthalate, tetraallylterephthalamide,tri(diallylamine)-s-triazine, triallyl phosphite, andN,N-diallylacrylamide. The particular component (f) range is selectedbecause if component (f) is present in amounts less than 0.1 part byweight, crosslink density is unacceptable. On the other hand, ifcomponent (f) is present in amounts above 10 parts by weight, surfaceblooming of component (f) occurs during molding, resulting in poor moldcharacteristics. The preferable range of component (f) is 0.2-6 parts byweight per 100 parts fluoroelastomer. The component (f) polyfunctionalunsaturated compounds may be used singly or as a combination of two ormore types.

The shrinkage of the fluoroelastomers of this invention which occursduring milling has been found to be low.

The vulcanized product prepared as described above exhibits low tensilestress at 100% elongation, low hardness, good elongation, and lowcompression set, thereby exhibiting elastomeric characteristics.

By carrying out vulcanization as described above, it is possible toprepare various molded packings and gaskets such as o-rings, V-packing,U-packing, Y-packing, D-rings, triangle rings, T-rings, X-rings, andrubber valve seats, butterfly valves, stem valves, oil seals, SF classengine oil resistant moldings, SG class engine oil resistant moldings,fuel hose, filler hose, in-tank hose, thermal contraction tubes, wetfriction materials, electric wire covering, piezoelectric material, andchimney duct joint bellows.

EXAMPLES

The invention is further illustrated by, but is not limited to, thefollowing examples.

Various physical properties of the fluoroelastomers of this inventionwere determined according to the following methods.

(1) The limiting viscosity η! was determined by dissolving a sample ofthe fluoroelastomer in methyl ethyl ketone to obtain a 0.1 g/100 mlsolution, which was used in a capillary viscometer in measurements at35° C.

(2) The molecular weight distribution was measured using liquidchromatography. A Model HLC-8020 chromatograph manufactured by Toso(K.K.)! was used, column: KF-80M (2 columns) plus KF-800P (precolumn)manufactured by Showa Denko K.K.)!, integrator: Model AS-8010manufactured by Toso (K.K.)!, development solvent: tetrahydrofuran,concentration 0.1 wt %, temperature: 35° C., and standard polymers formolecular weight working curve preparation: various polystyrene simpledispersion manufactured by Toso (K.K.), M_(w) /M_(n) of about 1.2(max)!.

(3) The Mooney viscosity was determined according to the method ofJIS-K6300 by using a Kamishima Seisakusho rotary viscometer, ModelVR-103ST with and L type rotor under conditions of 121° C., preheatingtime of one minute and rotor operation time of 10 minutes.

(4) The hardness of vulcanized products was measured according to themethod of JIS-A, and the tensile characteristics such as tensile stress,tensile strength, and tensile elongation were measured according to themethod of JIS-K6301.

(5) The compression set of the vulcanized products was measuredaccording to the method of JIS-6301 by using and o-ring P-24 defined inJIS-B2401, temperature of 200° C., and a time of 72 hours.

Example 1

A 15-liter autoclave, equipped with an electromagnetic stirrer wasblanketed with nitrogen gas. Subsequently, a solution prepared bydissolving 3.6 g of methyl cellulose (viscosity of 50 cp) as an emulsionstabilizer in 4760 g of deoxygenated pure water was added under vacuumto the autoclave. The stirrer speed was 600 rpm and the temperature wasmaintained at 50° C. A monomer mixture comprising 29.5 wt % of VF₂, 64.5wt % HFP, and 6.0 wt % TFE was added as an initial monomer mixture untilthe pressure reached 26.5 kg/cm² G. A polymerization initiator solutionwas then prepared by dissolving 4.3 g of the oil soluble organicperoxide IPP in 50.6 g of methyl acetate (a water soluble hydrocarbonsolvent). This solution was added to the autoclave under pressure andafter the polymerization was initiated, the pressure was reduced to 25.5kg/cm² G and a monomer mixture comprising 52.7 wt % VF₂, 25.5 wt % HFP,and 21.8 wt % TFE was added as a supplemental monomer mixture until thepressure reached 26.5 kg/cm² G. The polymerization reaction was carriedout in a continuous manner for 6 hours by repeating these procedures.After completion of the polymerization reaction, the remaining monomermixture was discharged, the suspension was dewatered in a centrifuge,and the solids obtained were thoroughly washed with water and dried at100° C. under vacuum to obtain about 6 kg of an elastomer. Thefluoroelastomer thus prepared was analyzed for monomer ratio using ¹⁹F-NMR and the VF₂ content was determined to be 54.6 wt %, the HFPcontent was determined to be 25.1 wt %, and the TFE content wasdetermined to be 20.3 wt %. In addition, the limiting viscosity η! ofthe fluoroelastomer was 258 ml/g and the Mooney viscosity ML₁₊₁₀ (121°C.) was 120.

One hundred parts by weight of the fluoroelastomer were milled on arubber mill with 20 parts of Cancarb medium thermal carbon black(Thermax N-990), 6 parts by weight of "Calbit" calcium hydroxide (OhmiChemical), 3 parts by weight "Kyowamag" high activity magnesium oxide(Kyowa Chemical Industries), 2 parts by weight of bisphenol AF and 0.3parts by weight of bis(benzyldiphenylphosphine)iminium chloride. Themilled product was allowed to age by standing overnight.

After milling, the mixture was placed in molds and press cured at 177°C. for 30 minutes to obtain a 2 mm thick sheet and o-ring. The curedpolymer was removed from the molds and heated in a hot air convectionoven at 232° C. for 24 hours to post cure the samples. The cured polymerwas tested and the physical properties are shown in Table 1.

Comparative Example 1

The atmosphere of a 15 liter autoclave equipped with an electromagneticstirrer was substituted with nitrogen by three times repeating theprocess of vacuum evacuation followed by feeding in nitrogen. A solutionof 4.64 g of methyl cellulose (viscosity 50 cp) dissolved in 4640 g ofdeoxygenated pure water was added to the autoclave under vacuum withstirring at 600 rpm while maintaining the autoclave temperature at 50°C. A monomer mixture comprising 5 28.9 wt % VF₂, 65.2 wt % HFP, and 5.9wt % TFE was added as an initial monomer mixture until the pressurereached 20 kg/cm² G. A polymerization initiator solution prepared bydissolving 8.6 g of the oil soluble organic peroxide IPP in 85.4 g ofCFC R-141b (CH₃ CFCl₂) was added under pressure to initiate thepolymerization reaction. After polymerization was initiated, thepressure was reduced to 19.5 kg/cm² G and a monomer mixture comprising52.7 wt % VF₂, 25.5 wt % HFP, and 21.8 wt. TFE was added as asupplemental monomer mixture until the pressure reached 20 kg/cm² G. Thepolymerization reaction was carried out in a continuous manner for 5hours with pressure being repeatedly boosted to 20 kg/cm² G by additionof supplemental monomer when the pressure fell to 19.5 kg/cm² G. Afterthe polymerization reaction was complete, the remaining monomer mixturewas discharged, the suspension was dewatered in a centrifuge and thesolids obtained were thoroughly washed with water and dried at 100° C.under vacuum to obtain about 4.5 kg of an elastomer. The fluoroelastomerproduct was analyzed for monomer ratio using ¹⁹ F-NMR. The content ofVF₂ units was 52.6 wt %, the HFP content was 26.5 wt % and the TFEcontent was 20.9 wt %. The limiting viscosity 72 ! of thefluoroelastomer was 257 ml/g and the Mooney viscosity ML₁₊₁₀ (121° C.)was 120. The product was vulcanized using the same method as describedfor Example 1. The physical properties of the cured products are shownin Table 1. The results obtained showed the same polymer properties andvulcanized properties as those obtained in Example 1. However, thesolvent R-141b used for the catalyst is ozone-destroying and is notsuitable for environmental reasons.

Example 2

The atmosphere of a 15 liter autoclave equipped with an electromagneticstirrer was substituted with nitrogen by three times repeating theprocess of vacuum evacuation followed by feeding in nitrogen. A solutionof 0.3 g of methyl cellulose (viscosity 50 cp) dissolved in 360 g ofdeoxygenated pure water was added to the autoclave under vacuum withstirring at 600 rpm while maintaining the autoclave temperature at 50°C. A monomer mixture comprising 29.9 wt % VF₂, and 70.1 wt % HFP wasadded as an initial monomer mixture until the pressure reached 13 kg/cm²G. A polymerization initiator solution prepared by dissolving 1.8 g ofthe oil soluble organic peroxide IPP in 4.0 g of methyl acetate wasadded under pressure to initiate the polymerization reaction. Afterpolymerization was initiated, the pressure was reduced to 12 kg/cm² Gand a monomer mixture comprising 63.1 wt % VF₂, and 36.9 wt % HFP wasadded as a supplemental monomer mixture until the pressure reached 13kg/cm² G. The polymerization reaction was carried out in a continuousmanner for 13 hours with pressure being repeatedly boosted to 13 kg/cm²G by addition of supplemental monomer when the pressure fell to 12kg/cm² G. After the polymerization reaction was complete, the remainingmonomer mixture was discharged, the suspension was dewatered in acentrifuge and the solids obtained were thoroughly washed with water anddried at 100° C. under vacuum to obtain about 300 g of an elastomer. Thefluoroelastomer product was analyzed for monomer ratio using ¹⁹ F-NMR.The content of VF₂ units was 65.3 wt %, and the HFP content was 34.7 wt%. The limiting viscosity η! of the fluoroelastomer was 90 ml/g

Example 3

The same method as that used in Example 2 was repeated except that 3.7 gof t-butyl acetate instead of 4.0 g of methyl acetate was used, and thepolymerization time was about 14 hours. The yield of the fluoroelastomerproduct was about 340 g, the monomer content of the polymer was 63.2 wt% VF₂ units and 34.7 wt % HFP units, and the limiting viscosity η! ofthe fluoroelastomer was 92 ml/g.

Comparative Example 2

The same method as that used in Example 2 was repeated except that 5.7 gof perchloroethylene instead of 4.0 g of methyl acetate was used. Theyield of the fluoroelastomer prepared was about 3 g, the monomer contentof the polymer was 63.5 wt % VF₂ units and 36.5 wt % HFP units, and thelimiting viscosity η! of the fluoroelastomer was 10 ml/g. The yield wasextremely low compared with that of Example 2 or 3, and the viscosity η!was extremely low.

Comparative Example 3

The same method as used in Comparative Example 1 was repeated under theconditions shown in Table 2. The properties of the polymer prepared areshown in Table 2. The solvent R-225cb used in the example isozone-destroying and it is not environmentally suitable.

Example 4

A 15 liter autoclave having an electromagnetic stirrer was flushed threetimes with nitrogen gas under vacuum. A solution of 4.1 g of methylcellulose (viscosity 50 cp) dissolved in 5.4 kg of deoxygenated purewater was added to the autoclave under vacuum with stirring at 600 rpmwhile maintaining the autoclave temperature at 50° C. A monomer mixturecomprising 15.6 wt % VF₂, 78.2 wt % HFP, and 6.2 wt % TFE was added asan initial monomer mixture until the pressure reached 24 kg/cm² G. Apolymerization initiator solution prepared by dissolving 4.3 g of theoil soluble organic peroxide IPP in 50.6 g of methyl acetate along with27.7 g diiodomethane was added under pressure to initiate thepolymerization reaction. After polymerization was initiated, thepressure was reduced to 23 kg/cm² G and a monomer mixture comprising47.7 wt % VF₂, 31.4 wt % HFP, and 20.9 wt % TFE was added as asupplemental monomer mixture until the pressure reached 24 kg/cm² G. Thepolymerization reaction was carried out in a continuous manner for 14.5hours by repeating these procedures. After the polymerization reactionwas complete, the remaining monomer mixture was discharged, thesuspension was dewatered in a centrifuge and the solids obtained werethoroughly washed with water and dried at 100° C. under vacuum to obtainabout 5 kg of an elastomer. The fluoroelastomer product was analyzed formonomer ratio using ¹⁹ F-NMR. The content of VF₂ units was 46.3 wt %,the HFP content was 32.5 wt %, and the TFE content was 21.2 wt %. Thelimiting viscosity η! of the fluoroelastomer was 50 ml/g and the Mooneyviscosity ML₁₊₁₀ (121° C.) was 7.

One hundred parts by weight of the fluoroelastomer was milled on arubber mill with 20 parts of MT carbon black (Thermax N-990) and 3 partsby weight "Kyowamag" high activity magnesium oxide (Kyowa ChemicalIndustries). Subsequently, 4 parts by weight of triallyl isocyanurate(TAIC) from Nippon Chemical Co. and 3.75 parts by weight of2,5-dimethyl-2,5-di(t-butylperoxy)hexane ("Perhexa 25B-40", peroxidecontent of 40 wt %, available from Nippon Oil and Fat Co.) was milledinto the polymer mixture. The milled product was allowed to age bystanding overnight.

After milling, the mixture was placed in molds and press cured at 177°C. for 30 minutes to obtain a 2 mm thick sheet and o-ring. The curedpolymer was removed from the molds and heated in a hot air convectionoven at 180° C. for 4 hours to post cure the samples. The cured polymerwas tested and the physical properties are shown in Table I.

Comparative Example 4

The same method as that used in Example 4 was employed except that thepolymerization initiator used was different. The polymer properties ofthe fluoroelastomer product and the cured polymer properties are shownin Table I. The results obtained are comparable to those obtained inExample 4, but the solvent used was ozone destroying and is harmful tothe environment.

Example 5

The atmosphere of a 10 liter autoclave equipped with an electromagneticstirrer was substituted with nitrogen by three times repeating theprocess of vacuum evacuation followed by feeding in nitrogen. A solutionof 4.16 g of methyl cellulose (viscosity 50 cp) dissolved in 4160 kg ofdeoxygenated pure water was added to the autoclave under vacuum withstirring at 600 rpm while maintaining the autoclave temperature at 50°C. A monomer mixture comprising 28.7 wt % VF₂, 65.6 wt % HFP, and 5.7 wt% TFE was added as an initial monomer mixture until the pressure reached20 kg/cm² G. A polymerization initiator solution prepared by dissolving10.3 g of the oil soluble organic peroxide IPP in 37.0 g of methylacetate was added under pressure to initiate the polymerizationreaction. After polymerization was initiated, the pressure was reducedto 19.5 kg/cm² G and a monomer mixture comprising 52.8 wt % VF₂, 25.4 wt% HFP, and 21.8 wt % TFE was added as a supplemental monomer mixtureuntil the pressure reached 20 kg/cm² G. The polymerization reaction wascarried out in a continuous manner for 4.5 hours by boosting thepressure to 20 kg/cm² G by addition of supplemental monomer when thepressure fell to 19.5 kg/cm² G. After the polymerization reaction wascomplete, the remaining monomer mixture was discharged, the suspensionwas dewatered in a centrifuge and the solids obtained were thoroughlywashed with water and dried at 100° C. under vacuum to obtain about 5 kgof an elastomer. The fluoroelastomer product was found to contain 51.7wt % VF₂, 28.0 wt % HFP, 20.3 wt % TFE units. The limiting viscosity η!of the fluoroelastomer was 120 ml/g and the number average molecularweight M_(n) was 11.1×10⁴.

The fluoroelastomer product was cured using the same method as used inExample 1to obtain a cured product whose properties were measured. Theresults obtained for polymer properties are shown in Table 4, and theproperties of the cured products are shown in Table 5.

Examples 6-9 and Comparative Examples 5-10

The same method as used in Example 5 was used except that the methylacetate was substituted with the solvents shown in Table 4 to dilute theIPP. The polymer properties measured are shown in Table 4. The samecuring method as used in Example 1 was used for the polymers prepared inExamples 6-9 to obtain cured molded products which were tested. Theproperties of these cured molded products are shown in Table 5.

Comparative Example 11

The same method as that used in Example 5 was used except that nosolvent at all was employed. Polymer properties are shown in Table 4.The same curing process as that used in Example 1 was used to obtaincured, molded products of the fluoroelastomer. The properties of thecured, molded products are shown in Table 5. The cured, molded productsof Comparative Example 11 were comparable to those prepared in Examples5-9 with respect to their physical properties. The procedures used arethose suitable for bench scale handling of IPP without solvent dilution,but if scaled up to an industrial scale, the decomposition of theperoxide becomes a problem, and consequently, the method is notpractical.

                  TABLE 1                                                         ______________________________________                                                          Example (PE)                                                                  or comparative example (CE)                                                   PE 1 CE 1    PE 4   CE 4                                    ______________________________________                                        Polymer characteristics                                                       Composition                                                                   VF.sub.2 unit       54.6   52.6    46.3 47.5                                  HFP unit            25.1   26.5    32.5 31.4                                  TFE unit (wt %)     20.3   20.9    21.2 21.1                                   η! (ml/g)      258    257     50   50                                    Number average m.w. Mn (× 10.sub.4)                                                         25.6   28.5    4.2  4.8                                   Weight average m.w. Mw (× 10.sub.4)                                                         90.2   96.7    8.6  9.6                                   Mooney viscosity    120    120     7    9                                     ML.sub.1+10 (121° C.)                                                  Vulcanized product characteristics                                            Hardness (pts)      68     68      73   73                                    100% Tensile stress (kgf/cm.sup.2)                                                                43     45      52   49                                    Tensile strength (kgf/cm.sup.2)                                                                   149    148     212  211                                   Elongation (%)      245    245     300  305                                   Compression set (%) 9      9       24   22                                    Ozone destruction coefficient of the                                                              0      0.11    0    0.8                                   solvent used for the initiator.sup.1                                          ______________________________________                                         .sup.1 Ozone destruction coefficient cited from UNEP Synthesis Report         1991.                                                                    

                  TABLE 2                                                         ______________________________________                                                                 Comparative                                                                   Example No.                                                                   3                                                    ______________________________________                                        Polymerization conditions                                                     Initally charged monomer composition                                          VF.sub.2                   30.0                                               HFP (wt %)                 70.0                                               Supplemental monomer composition                                              VF.sub.2                   63.1                                               HFP (wt %)                 36.9                                               Polymerization temperature (°C.)                                                                  50                                                 Polymerization pressure (kg/cm.sup.2 G)                                                                  13                                                 Amount of pure water charged (kg)                                                                        5.4                                                Methylcellulose (g)        5.4                                                Polymerization  IPP.sup.1 (g)  26                                             initiatior      Solvent (g)    R-225cb.sup.2                                  solution        Ozone destruction coeff..sup.3                                                               0.025                                          Polymerization time (hr)   13                                                 Elastomer yield (g)        5,100                                               η! (ml/g)             85                                                 Polymer composition                                                           VF.sub.2 unit              62.9                                               HFP unit (wt %)            37.1                                               Mooney viscosity ML.sub.1+10 (121° C.)                                                            44                                                 ______________________________________                                         .sup.1 IPP: dissopropyl peroxydicarbonate                                     .sup.2 R225cb: CClF.sub.2 CF.sub.2 CHClF                                      .sup.3 Ozone destruction coefficient cited from UNEP Synthesis Report         1991.                                                                    

                  TABLE 3                                                         ______________________________________                                                                 COMPARATIVE                                                          EXAMPLE 4                                                                              EXAMPLE 4                                            ______________________________________                                        Polymerization conditions                                                     Initally charged monomer                                                      composition                                                                   VF.sub.2          15.6       15.6                                             HFP               78.2       78.2                                             TFE (wt %)        6.2        6.2                                              Supplemental monomer composition                                              VF.sub.2          47.7       47.7                                             HFP               31.4       31.4                                             TFE (wt %)        20.9       20.09                                            Polymerization temperature (°C.)                                                         50         50                                               Polymerization pressure (kg/cm.sup.2 G)                                                         24         24                                               Amount of pure water charged (kg)                                                               5.4        5.4                                              Methyl Cellulose) 4.1        4.1                                              Diiodomethane (g) 27.7       27.7                                             Polymerization                                                                         IPP.sup.1 (g)                                                                              4.3        4.3                                          initiator                                                                              Solvent (g)  Methyl acetate                                                                           R-113.sup.2                                  solution              50.6       86                                                    Ozone destruction                                                                          0          0.8                                                   coeff.sup.3                                                          Polymerization time (hr)                                                                        14.5       13                                               Elastomer yield (g)                                                                             About 5    About 5                                          ______________________________________                                         .sup.1 IPP: diisopropyl peroxydicarbonate                                     .sup.2 R113: CCl.sub.2 FCClF.sub.2                                            .sup.3 Ozone destruction coefficient cited from UNEP Synthesis Report         1991.                                                                    

                                      TABLE 4                                     __________________________________________________________________________                               Polymer characteristics                            Solvent                    Limit Number avg.                                                                          Composition                                      Amount                                                                            Initiator solution                                                                    Yield                                                                             viscosity  η!                                                                   molecular wt.                                                                        VF.sub.2 /HFP/TFE                     Type       (g) concn. (wt %)                                                                         (kg)                                                                              (ml/g)                                                                              Mn     (wt %)                                __________________________________________________________________________    PE 5                                                                              Methyl acetate                                                                       37  21.8    About                                                                             120   11.1 × 10.sup.4                                                                51.7/28.0/20.3                                               2.5                                                    PE 6                                                                              Methyl formate                                                                       30  25.6    About                                                                             114    8.9 × 10.sup.4                                                                52.4/27.1/20.5                                               2.2                                                    PE 7                                                                              Methyl alcohol                                                                       16  39.2    About                                                                             171   14.0 × 10.sup.4                                                                52.7/26.8/20.5                                               2.5                                                    PE 8                                                                              t-Butyl alcohol                                                                      37  21.8    About                                                                             176   14.4 × 10.sup.4                                                                53.5/25.9/20.6                                               3.3                                                    PE 9                                                                              Acetone                                                                              29  26.2    About                                                                              91    8.1 × 10.sup.4                                                                52.0/27.6.20.4                                               2.0                                                    CE 5                                                                              Ethyl acetate                                                                        44  19.0    About                                                                             54     3.4 × 10.sup.4                                                                52.4/27.1/20.5                                               1.4                                                    CE 6                                                                              Ethyl alcohol                                                                        23  30.9    ABout                                                                             71     5.5 × 10.sup.4                                                                51.0/27.6/21.4                                               1.1                                                    CE 7                                                                              sec-Butyl                                                                            37  21.8    About                                                                             23     0.9 × 10.sup.4                                                                51.1/30.9/18.0                            alcohol            0.1                                                    CE 8                                                                              Methyl ethyl                                                                         36  22.2    About                                                                             36     0.8 × 10.sup.4                                                                52.6/29.6/17.8                            ketone             0.0                                                    CE 9                                                                              Methyl t-butyl                                                                       44  19.0    About                                                                             21     0.6 × 10.sup.4                                                                49.4/31.6/19.0                            ether              0.1                                                    CE 10                                                                             Benzene                                                                              39  20.9    No polymerization                                      CE 11                                                                             None   --  --      About                                                                             192   17.3 × 10.sup.4                                                                54.1/26.6/19.3                                               2.6                                                    __________________________________________________________________________     PE: practical example, CE: comparative example                           

                                      TABLE 5                                     __________________________________________________________________________               Hardness                                                                           100% Tensile                                                                          Tensile Strength                                                                      Elongation                                                                         Compression                              Solvent    (pts)                                                                              Stress (kgf/cm2)                                                                      (kgf/cm2)                                                                             (%)  Set (%)                                  __________________________________________________________________________    PE 5                                                                              Methyl acetate                                                                       57   39      159     250  13                                       PE 6                                                                              Methyl formate                                                                       67   38      160     260  13                                       PE 7                                                                              Methyl alcohol                                                                       68   42      165     245  11                                       PE 8                                                                              t-Butyl alcohol                                                                      68   44      165     245  14                                       PE 9                                                                              Acetone                                                                              68   40      149     245  16                                       CE 11                                                                             None   68   45      167     255  13                                       __________________________________________________________________________     PE: practical example, CE: comparative example                           

We claim:
 1. A suspension process for producing a fluoroelastomer havingcopolymerized units of vinylidene fluoride monomer and at least oneother copolymerizable fluorinated monomer which comprises(A) dispersingsaid monomers in an aqueous medium containing 0.001-3 parts by weight ofa suspension stabilizer per 100 parts of the aqueous medium and 0.001-5parts by weight of an oil soluble organic peroxide polymerizationinitiator solution per 100 parts of the aqueous medium; and (B)polymerizing the resultant dispersion at a temperature of 45° C.-70°C.;wherein said oil soluble organic peroxide polymerization initiatorsolution consists essentially of 0.1-75 wt. % of an oil soluble organicperoxide in a water-soluble hydrocarbon solvent and said water-solublehydrocarbon solvent contains no halogen atom and is represented bycompounds of the formulas (C₃)₃ COH, R₂ COOR₁, and R₁ COR₃, where R₁ andR₃ are methyl or t-butyl groups, and R₂ is hydrogen, a methyl group or at-butyl group.
 2. The process of claim 1 wherein the suspensionpolymerization is carried out in the presence of 0.005-5 parts by weightper 100 parts by weight of the aqueous medium of an iodo compoundrepresented by the formula

    RI.sub.n,

where R is a hydrocarbon group having 1-3 carbon atoms or saturated orunsaturated fluorohydrocarbon or chlorofluorohydrocarbon group havingless than 6 carbon atoms, and n is 1 or
 2. 3. The process of claim 1wherein the fluoroelastomer produced comprises copolymerized units ofvinylidene fluoride and hexafluoropropylene.
 4. The process of claim 1wherein the fluoroelastomer produced comprises copolymerized units ofvinylidene fluoride, hexafluoropropylene, and tetrafluoroethylene. 5.The process of claim 1 wherein the fluoroelastomer produced comprisescopolymerized units of 10-85 mol % of vinylidene fluoride, 3-80 mol %tetrafluoroethylene, and 2-50 mol % of perfluoroalkyl perfluorovinylether.
 6. The process of claim 1 wherein the oil soluble organicperoxide is a dialkyl peroxydicarbonate.
 7. The process of claim 6wherein the oil soluble organic peroxide is diisopropylperoxydicarbonate.
 8. The process of claim 1 wherein the water-solublesolvent is a compound represented by the formula R₁ --OH, where R₁ is at-butyl group.
 9. The process of claim 1 wherein the water solublesolvent is t-butyl alcohol.
 10. The process of claim 1 wherein the watersoluble solvent is a compound represented by the formula R₂ --COO--R₁,where R₁ is a methyl or t-butyl group and R₂ is hydrogen, a methyl groupor a t-butyl group.
 11. The process of claim 1 wherein the water solubleorganic solvent is methyl acetate.
 12. The process of claim 1 whereinthe water soluble organic solvent is t-butyl acetate.
 13. The process ofclaim 1 wherein the water soluble solvent is a compound represented bythe general formula R₁ --CO--R₃, where R₁ and R₃ are methyl or t-butylgroups.
 14. The process of claim 1 wherein the polymerization is carriedout at a temperature of from 50°-60° C.
 15. The process of claim 2wherein the iodo compound is a compound represented by the formulaRI_(n), where R is a hydrocarbon group having 1-3 carbon atoms, and n is1 or 2 .
 16. The process of claim 15 wherein the iodo compound isdiiodomethane.